Negative refractive index materials (NIMs) have extraordinary promise in imaging and lithography applications. Unlike a conventional lens, a negative refractive index implies that when a material refracts an incoming light ray, the refracted ray will be deviated at a negative angle to the normal according to Snell's law, as shown in FIG. 1. This seemingly trivial observation has profound consequences because, as implied by the figure, focusing can be accomplished by a slab of material rather than a conventionally-shaped lens. More subtly, lenses made from NIMs can be much more compact than conventional lenses, and wave vector components along the optical axis can be used for imaging since evanescent waves grow in NIMs instead of decay. In contrast, these components decay at distances very close to the lens surface (the near field) in conventional optics accounting for a loss of imaging information and, ultimately, loss of resolution. Furthermore, a negative index implies that the phase of a wave decreases through NIMs rather than advances. A material with n=−1 can be considered to reverse the effect of propagation through an equivalent thickness of vacuum. Consequently, negative index materials have a potential advantage of forming highly efficient low reflectance surfaces by canceling the scattering properties of other materials, and if the material is isotropic, these effects occur regardless of the direction of the incident wave creating what theoreticians have called this the perfect lens: a planar slab, without reflective losses, which could focus both the propagating and evanescent components of an object and achieve sub-wavelength imaging.
Negative index of refraction metamaterials can be obtained when the electrical permittivity (∈) is less than 0 and magnetic permeability (μ) is less than 0. Typically these are achieved when there is resonance behavior in the material. Electrical resonances are common in metals at optical frequencies, but magnetic resonant conditions do not occur in natural materials at these frequencies, therefore the construction of metamaterials involves engineering a negative permeability material using non-magnetic materials. At radio frequencies, this can be done using small (mm-scaled) metallic inclusions to achieve negative effective value for μ to create structures that have a negative index of refraction in certain microwave bands. These are usually ring-shaped strips of metal, and the magnitude of the magnetic moment that forms from the induced current becomes large (positive or negative) under resonance conditions. More recently, it has been possible to lithographically define tiny optical-frequency resonators in materials, which have resulted in negative index behavior in the visible and near-infrared spectrum. Still, these materials are (1) not isotropic, i.e., the features are planar, and the index varies with orientation, and (2) most of these materials exhibit large optical losses.
Properties that can be exploited using NIMs include:
Reversal of Snell's Law: When light passes through NIMs, the sign on the relationship between incident and refracted light rays changes. Therefore, light is refracted away from the normal instead of toward it. This allows devices such as a flat lens, which may lead to more compact optics, and in curved lenses, NIMs may focus light to a much shorter focal length using less material. There is not a near-field limitation implied with the reversal of Snell's law.
Evanescent Field Grows Instead of Decays: A very interesting consequence of negative index is that the evanescent field within the light will grow exponentially instead of decay as in conventional lenses. As illustrated in FIG. 2, evanescent waves may originate from three interfaces: the object, where it decays; the front of the lens at the aperture, where it begins to grow; and from the back of the lens, where it again decays. In an NIM lens, optical information at the interface is not lost, but can be reconstructed at the image plane. In principle, the resolution of the reconstructed image may be better than the diffraction limit creating a phenomenon known as super-resolution.
A super-resolution lens can be constructed so that the information at the aperture is completely reconstructed, such that there is improved imaging at the plane of the detector of the light that is received. This may create a smaller spot size eliminating some limitations of conventional lenses and allowing for smaller pixels in the detector (or that finer features may be obtained in optical lithography). The image plane created by a NIM lens is in the near field, which is applicable in some applications, but techniques are being developed to project the information at the aperture onto an image plane in the far field as well, e.g., with a grating coupled to the NIM.
NIM properties vary with wavelength: For most applications, a broad band of negative index behavior is preferred, but these are still designed around resonances, and in some cases, a very narrow bandwidth may be desired. Under these conditions, transmission and reflection properties will go through extremes of near-perfect transmission or reflection, and at the proper wavelength can be designed to have negligible reflectance at wavelengths of interest, which is desired in certain optical systems.